Yes the benefit of an interest-free loan from a non-UK trust is a deemed capital payment. Its UK tax treatment will depend on the composition of the trust and whether there are any stockpiled gains, offshore income gains or relevant income in the trust.
Normally the ‘benefit’ calculated using HMRC rates will crystalise, in the hands of a UK resident beneficiary (i.e. be matched against), an equivalent amount of stockpiled gains. If the amount exceeds the beneficiary’s annual exempt amount then he will be subject to UK capital gains tax thereon. If the matched gains are more than a year old there will be a 10% surcharge, although newer gains are matched in priority to older gains (since 2008).
However, if there are any stockpiled offshore income gains or relevant (undistributed) income then these take precedence and the benefit will be treated as income in the hands of the UK beneficiary.
Also the loan will be a UK situs asset of the trust that may bring it within the scope of UK inheritance tax, if it wasn’t already.
This is a fairly complex area and the rules have changed several times over recent years. In the current climate of HMRC paranoia about all things offshore it is worth taking proper specialist advice that fully considers all the relevant circumstances.